The present invention relates to an internal magnetic shield provided in a cathode ray tube to reduce mislanding of an electron beam due to an external magnetic field such as geomagnetism, and to a cathode ray tube including the same.
FIG. 11 shows a conventional cathode ray tube used in television receivers, computer displays, or the like. An electron beam 81 released from an electron gun 80 is deflected in vertical and horizontal directions by a deflection yoke 82 to scan the entire screen, so that images are reproduced. In this case, when the cathode ray tube is affected by an external magnetic field such as geomagnetism, the path of the electron beam 81 is distorted. Therefore, the electron beam 81 does not reach the desired position on a phosphor screen 84 formed on a front panel 83, resulting in mislanding. To deal with this problem, the cathode ray tube includes an internal magnetic shield 85 that provides a shield against geomagnetism or the like.
As shown in FIG. 12, the internal magnetic shield generally includes a pair of opposing long side walls 86, a pair of opposing short side walls 87, and an opening 88 formed in the center, or substantially V-shaped notches 89 formed on the short side walls 87 as shown in FIG. 13. Such V-shaped notches 89 are disclosed in JP 53 (1978)-15061 A, JP 7 (1995)-192643 A, JP 5 (1993)-159713A, or the like.
When a cathode ray tube, including the internal magnetic shield without notches on the short side walls 87 or with substantially V-shaped notches, is affected by an external magnetic field such as geomagnetism, the amount of mislanding tends to be larger in the periphery of the screen than in the center thereof. In particular, mislanding occurs significantly at the corners, i.e., edges, of the screen. Thus, the conventional internal magnetic shields cause non-uniform mislanding throughout the screen, so that the improvement of mislanding at the corners of the screen has been necessary, particularly for a cathode ray tube that requires high definition.
It is not preferable that the amount of mislanding varies depending on the direction in which the cathode ray tube is oriented. To avoid this, it is preferable that the amount of mislanding due to geomagnetism in the tube-axis direction is substantially the same as that of mislanding due to geomagnetism in the horizontal direction perpendicular to the tube axis. However, it is difficult to reduce the amount of mislanding throughout the screen while achieving the balance between two mislandings by geomagnetism in different directions.
Therefore, with the foregoing in mind, it is an object of the present invention to provide an internal magnetic shield that can reduce mislanding of a deflected electron beam by an external magnetic field such as geomagnetism and prevent the displacement and unevenness of colors on the entire screen. It is another object of the present invention to provide an internal magnetic shield that easily can balance the amount of mislanding due to geomagnetism in the tube-axis direction and in the horizontal direction perpendicular to the tube axis while reducing mislanding throughout the screen. It is yet another object of the present invention to provide a cathode ray tube that can display favorable images with reduced displacement and unevenness of colors on the entire screen by including the above internal magnetic shield.
To achieve the above objects, a first internal magnetic shield for a cathode ray tube of the present invention includes a pair of opposing long side walls, a pair of opposing short side walls, and an opening enclosed by these side walls in the center. At least one pair of the long and short side walls are provided with notches having a substantially home-plate shape.
A second internal magnetic shield for a cathode ray tube of the present invention includes a pair of opposing long side walls, a pair of opposing short side walls, and an opening enclosed by these side walls in the center. At least one pair of the long and short side walls are provided with notches. Each of the notches is formed by at least two pairs of opposing cutting edges with different orientations.
The above first and second internal magnetic shields can reduce mislanding of a deflected electron beam by an external magnetic field such as geomagnetism and prevent the displacement and unevenness of colors on the entire screen. Moreover, they easily can balance the amount of mislanding due to geomagnetism in the tube-axis direction and in the horizontal direction perpendicular to the tube axis while reducing mislanding throughout the screen.
A cathode ray tube of the present invention includes an envelope having a front panel and an funnel, a phosphor screen formed on the inner surface of the front panel, a color selection electrode arranged to face the phosphor screen, an electron gun placed in the funnel, and an internal magnetic shield placed between the color selection electrode and the electron gun. The internal magnetic shield is the magnetic shield according to the above first or second internal magnetic shield.
The above cathode ray tube can display favorable images with reduced displacement and unevenness of colors on the entire screen, regardless of the direction in which the cathode ray tube is oriented.